1. Field of the Invention
The present invention relates to an electrically heated catalytic converter system for an engine that has a catalytic converter equipped with an electric heater.
2. Description of the Related Art
An exhaust gas purification device that utilizes a three-way reducing and oxidizing catalytic converter disposed on the exhaust passage of the internal combustion engine is commonly used. Generally, the catalyst used in such converters is able to purify pollutants in the exhaust gas only when the temperature of the catalyst becomes higher than a certain temperature, i.e., the catalysts in the converter do not work when the temperature is lower than the activating temperature of the catalyst.
Usually, the catalyst in the converter is gradually heated by the exhaust gas and reaches the activating temperature after the engine starts. However, when the temperature of the engine is low, for example, after a cold start of the engine, it takes a long time to heat the catalyst to the activating temperature since the heat of the exhaust gas is removed by the cold wall of the exhaust passage before it reaches the converter. Therefore, in a cold start of the engine, the exhaust gas of the engine is not sufficiently purified since the temperature of the catalyst is lower than the activating temperature.
To solve this problem, Japanese Unexamined Utility Model Publication (KOKAI) No. 47-22313 discloses a catalytic converter equipped with an electric heater for heating the catalyst after the engine starts. In this device, the electric heater is turned on for a predetermined time period, after the engine starts, to heat the catalyst so that the temperature of the catalyst reaches the activating temperature in a short time. When the predetermined time lapsed after the engine starts, the electric heater is turned off. However, the temperature of the catalyst reaches the activating temperature before the electric heater is turned off, and an oxidizing reaction starts in the catalytic converter. Also, the exhaust gas temperature increases when the predetermined time has lapsed. Therefore, the catalyst is maintained at high temperature by the heat given by the exhaust gas as well as by the heat generated by the oxidation of the HC and CO components in the exhaust gas by the catalyst.
However, in order to heat the catalytic converter in a short time to the activating temperature, it is required to supply a large electric current to the electric heater. In some cases, this causes the contacts of the switching device to be welded together due to the heat generated by the large electric current. When such a failure of the switching device occurs, the electric heater cannot be turned off after the predetermined time has lapsed. In this case, the electric current continues to flow through the electric heater even after the oxidation of HC and CO components starts in the catalytic converter, and the catalytic converter receives heat from the electric heater in addition to the heat from the exhaust gas and the heat generated by the oxidation of HC and CO components in the exhaust gas. If engine operation is continued in such conditions, the temperature of the catalytic converter becomes excessively high, and a deterioration of the catalyst or a damage of the substrate of the catalyst may occur due to the excessive temperature. Therefore, it is necessary to stop the engine when such failure of the switching device occurs.
However, in case of the automotive engines, it is preferable to continue the engine operation even in such failures in order to bring the vehicle to the service garage for repair.
It may be possible to continue the engine operation even when such failure occurs if an additional emergency switch for disconnecting the electric heater is provided to the heater circuit. However, even if such an emergency switch is provided, the damage of the catalytic converter is still possible if the contacts of the emergency switch are welded. Further, it is not preferable to add another switch to the circuit since it increases the complexity of the system and lowers the reliability of the system.